traffic_replay: set gensec features to encrypt credentials

traffic_replay: set gensec features to encrypt credentials

While running traffic_replay script against windows dc, it will fail
with a `LDAP_UNWILLING_TO_PERFORM` error for adding user.

Windows requires the credentials to be encrypted before sending.
`set_gensec_features` will fix it.

Signed-off-by: Joe Guo <joeg@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>